The invention relates to a method for machining a bore in a workpiece utilizing a numerically controlled machine tool. The tool comprises a boring bar rotatable and axially displaceable relative to the workpiece and a milling cutter rotatably mounted in the boring bar, the milling cutter having an axis of rotation radially offset with respect to the axis of the boring bar.
A tool of this type is disclosed in an article "Frasbohren und andere Bohrtechnologien" in the periodical "tz fur Metallbearbeitung", 81st annual, 1987, issue 3/87, pages 21, 22. The "bottle boring milling" tool shown therein comprises a rotating milling cutter mounted to the forward end of the stationary boring bar such that the milling cutter removes material from the workpiece in the area of the bore center when the boring bar is moved into the rotating workpiece. The boring cutting edge provided at the boring bar provides for the finish-machining. One advantage of this device which combines milling and boring operations resides in the fact that relatively short chips result ensuring good chip control. Furthermore, sufficiently high cutting speeds across the whole bore bottom are obtained. However, this bore milling tool is not capable of making profiled bores, i.e., bottle bores.
Applicant's German patent application No. 34 10 967 discloses a tool for making enlarged profiled bores. The application discloses a tool which comprises a rotating boring head having a plurality of circumferentially spaced cutting members adapted to be radially adjusted during the cutting operation by means of a single adjusting device. The tool involves merely a boring operation providing relatively long chips which may cause problems as to chip control. This may detrimentally affect both operational reliability and service life.
It is a primary object of the present invention to provide a tool adapted to make enlarged profiled bores while ensuring good chip control.
To this end, a tool of the above-defined type is characterized according to the present invention in that the milling cutter is mounted for radial movements relative to the boring bar.
Preferably, the milling cutter has cutting edges facing in the direction of removal of the boring bar and radially projecting beyond the outer periphery of the boring bar when the milling cutter is in its operative position.
The tool of the present invention allows both for axial and radial movements of the cutting edges of the milling cutter along a pair of NC axes. Said axial movements result from axial movements of the boring bar while said radial movements result from radial movements of the milling cutter relative to the boring bar. The tool is adapted to follow a profiled contour when the boring bar is removed from a preformed bore of the workpiece, with the milling cutter providing for the finish-machining of the profiled bore. Using a milling cutter results in relatively short chips ensuring good chip control. High operational reliability and long service life result therefrom.
Preferably, the boring bar is stationary while the workpiece rotates. However, relative movement between the boring bar and the workpiece could be obtained by making the boring bar rotate and holding the workpiece stationary.
The radial movements of the milling cutter relative to the boring bar are obtained either by means of an eccentric spindle having an axis of rotation radially offset relative to the axis of the boring bar and rotatable in the boring bar, or by means of a slide which is mounted to the boring bar such as to be radially movable relative thereto.